Good point, I'm not a very big fan of the term Dark Ages.
The last straw will likely be too late but it will be when enough people start to identify that something is wrong beyond a simple fix, and that we have all been hurt by it. Or if something like what's currently happening in Turkey were to happen.
It seems like the very idea of revolution is losing power.
Between the Arab Spring, Syrian Revolution (at least the start of it), and what's currently happening in Turkey I would have to disagree with you. As an additional note, the Egyptian Supreme Court ruled against the law that formed the parliament, and also ruled that the body that formed the constitution was illegal. It will be interesting to see if we have an Egyptian Revolution 2.0, depending on how the politicians react to this. Meanwhile, many parts of Africa are constantly in a rebellion, revolution, or other armed conflict.
But looking just at "major" 1st world countries such as the United States, Canada, most of Western Europe, etc. it will probably take much longer to reach that point. Things may not be great in these countries, but they are certainly better than Turkey or Africa and until we reach something like that, or more economic busts (maybe Student loans?) we won't see that last straw.